Apparatuses of the above kind are known. An apparatus with a linear analog sensor is described in European Patent Application No. 81 106 262 of the present applicant. This known apparatus is also represented in FIG. 1. The apparatus is denoted by 2 and contains an illumination unit 3, from which a light ray 14 emanates. This falls on an object of measurement 1. Reflected light ray 15 is projected onto the linear analog sensor 4 by an optical image reproduction system 5. If the distance of the object 1 increases from X.sub.0 to X.sub.1, then reflected ray 16 arrives on the linear analog sensor 4 through the optical image reproduction system 5. Each of the two reflected rays 15, 16 strikes the sensor 4 at a different point. Currents i.sub.1 and i.sub.2 produced by the sensor 4 are evaluated in an evaluation circuit 6 working in an analog manner for determining light spot position on the sensor 4. From this position the distance of the object 1 in relation to the apparatus 2 or the illumination unit 3 or the measurement plane of the sensor 4 can, in turn, be determined.
FIG. 2 shows the construction of the linear analog sensor 4 slightly more precisely. This consists of an elongated semiconductor chip 5 which is equipped with electrodes 6, 7 at both its ends. The measurement plane 8 extends between the electrodes 6,7. The electrodes are biassed relative to the semiconductor chip by a battery 13. The incident light rays 15 or 16 produce light spots 9 or 10 on the measurement plane 8. The light spot 9 is at distance (position) y.sub.i from the electrode 7. The light spot 10 is at distance (position) y.sub.k from the electrode 7. The currents i.sub.1 and i.sub.2 measured by current measuring apparatus 11, 12 depend on the position of the light spot on the measurement plane 8.
The linear analog sensor 4 shown in FIG. 2 has the advantage that it reacts to the slightest changes to the light spot on the measurement plane and because of this has a high resolution. On the other hand, it has the disadvantage that with incidence of spurious light it produces a false reading. If one imagines, for example, that the light spot 10 is created by an undesired reflected light ray, which also originates from the illumination unit 3, then the influence of the light spot 10 on the measuring result cannot be eliminated even if the light ray 14 emanating from the illumination unit 3 is modulated and demodulation is effected in the evaluation circuit 6 in FIG. 1. The linear analog sensor 4 reacts therefore to both light spots 9, 10 simultaneously, as it simply adds the minority carriers released by both light spots. The measuring result established in this way would lead to a falsely calculated position, which lies between y.sub.i and y.sub.k.
In order to avoid the disadvantage outlined last, a linear charge-coupled semiconductor sensor is used, as shown in FIGS. 4a and 4b, in place of the linear analog sensor. Such a linear semiconductor sensor is described, for example, in DE-PS 22 59 008. It is also known as a CCD-sensor (charge-coupled-device). FIG. 4a shows the linear charge-coupled semiconductor sensor 104 in section, while FIG. 4b shows it from above. It consists of an elongated semiconductor chip 112, which is covered by an insulating layer 113. On the insulating layer 113 there are arranged electrode triplets 114a to c and 115a to c. The corresponding electrodes of these electrode triplets are connected to each other by supply leads. A semiconductor zone 116 provided at the end is of a charge type which is opposite to that of the semiconductor body, so that a pn junction is formed between zone 116 and the semiconductor body 112. Furthermore the semiconductor zone 116 is connected to an electrode 117. The electrode 117 is connected by way of a direct-current source 118 to a resistor 119 which is connected to ground. The direct-current source 118 is poled in such a way that the pn-junction is reverse biassed. Charge carriers are released by light impinging on the semiconductor body 112, which carriers assemble as charge carrier packets close to the electrodes. By means of pulsating dc voltage applied to the electrodes from right to left the charge carrier packets are transported further in the same direction, whereby pulses appear on the resistor 119. In FIG. 4b a charge carrier packet assembled over the electrode 115c due to light spot 9 is indicated by dark dots. A charge carrier packet assembled over the electrode 115a due to the light spot 10 is indicated by less dense dark dots. Since the light spot 9 is more intense than the light spot 10, the charge carrier packet which assembles over the electrode 115c is denser than that which assembles over the electrode 115a. Consequently the pulses 121a to c and 122a to c appearing on the resistor 119 over time are of differing heights. The highest pulse 122a corresponds to the charge carrier packet assembled over the electrode 115c. The second highest pulse 122c corresponds to the charge carrier packet assembled over the electrode 115a. From the temporal separation of the pulses from the beginning of the pulse train, the position y.sub.i of the light spot 9 or the position y.sub.k of the light spot 10 can be determined.
From the representation of the pulse train of FIG. 4a one can recognise that it is possible, by means of a threshold voltage discriminator with a threshold voltage 120, to distinguish between the different height pulses 122a and 122c. In other words, it is possible in this way to suppress the pulse 122c derived from the spurious light spot 10 whilst the pulse 122a derived from the useful light spot is taken alone for evaluation.
An appropriate circuit which is also known is shown in FIG. 3. The same features as in FIG. 1 are denoted here by the same reference numbers. The apparatus is denoted by 102 and the linear charge-coupled semiconductor sensor by 104. The output of the charge-coupled semiconductor sensor 104 is connected to a threshold voltage discriminator 110, the output product of which is led to a circuit 111 for calculating the point of concentration. Thereby, the position of the light spot on the measurement plane can be established.
As compared with the linear analog sensor, the linear charge-coupled semiconductor sensor not only has the advantage that it opens up the possibility of efficiently distinguishing between spurious and useful light, even when the spurious light originates from the same illumination unit as the useful light, but it also has the further advantage of being more sensitive than the linear analog sensor. In other words, this means that the linear charge-coupled semiconductor sensor reacts to light of less intensity, to which the linear analog sensor no longer responds because of its own high background noise level. The linear charge-coupled semiconductor sensor, however, has, compared to the linear analog sensor, the disadvantage of having, for approximately the same dimensions, an appreciably smaller resolution. This is because it is not possible for technical-construction reasons to accommodate more than a certain number of electrodes on a specified length of the sensor. This can only be counteracted if the dimensions of the linear charge-coupled semiconductor sensor are increased appreciably in comparison with those of the linear analog sensor, whereby more electrodes can be arranged in a series. This also has the result, however, that the distance (focus) relative to the optical image reproduction unit and relative to the object has to be chosen essentially larger, which involves the disadvantage that the apparatus becomes altogether considerably larger. In this connection it should be mentioned that up to now linear charge-coupled semiconductor sensors have been known with which approximately 4000 electrodes are arranged in one series.